Dual beam laser ablation

ABSTRACT

A method for laser ablation of a tissue including directing at least one first pulsed laser beam of ultraviolet light and at least one second pulsed laser beam of infrared light to an area on the tissue to thereby ablate said tissue. The ultraviolet light has a wavelength of between 180 to 225 nm while the infrared light has a wavelength of between 1.4 to 11.0 μm, and more specifically around 3.0 μm. The tissue to be ablated may be the corneal tissue of the eye.

The present invention relates to the laser processing or ablation oftissue, in particular human or animal tissue. This invention isapplicable for use in operations on the corneal tissue of the eye forthe correction of myopia, myopic astigmatism, hyperopia and other visualproblems. These operations are known as photorefractive keratectomy orphototherapeutic keratectomy, and the present invention will thereforebe described with reference to its use in such an operation. It ishowever to be appreciated that other applications are also envisaged.

Currently this operation is performed using excimer lasers operating atwavelengths of 193 nm. The extremely high absorption of excimer laserbeams of this wavelength in corneal tissue, and the high photon energywhich can directly break molecular bonds therein allows this laser toablate with very high precision and with virtually no damage to theremaining tissue. However, excimer lasers are bulky and expensive lasersthat require significant maintenance and costs to run. These lasers alsouse a very corrosive gas.

There would therefore be significant benefits in replacing the excimerlaser with an alternative laser source. Solid state lasers are availablewhich can produce laser beams at reduced wavelengths of around 200 nm.However, these solid state lasers do not have enough energy to ablatetissue over a large enough area to be useful for this operation. Otherlasers are available, such as the Holmium or Erbium laser, which producea laser beam having very high absorption in corneal tissue and havingenough energy to ablate the tissue over a large enough area. However,because the photon energy at the infra-red wavelength of the laser beamproduced by these lasers is not high enough to directly break molecularbonds, the tissue adjacent to the ablated zone is damaged to too greatan extent to be useful for this operation.

It is therefore an object of the present invention to provide animproved method and apparatus for laser ablation of tissue.

With this in mind, according to one aspect of the present invention,there is provided a method for laser ablation of corneal tissue,including directing at least one first pulsed unfocused laser beam ofultraviolet light and at least one second pulsed unfocused laser beam ofinfrared light to an area on the tissue to thereby ablate said tissue.The use of pulsed laser beams facilitates the ablation of the tissuewhile minimising any damage to the surrounding tissue.

The pulses of each said at least one first and second laser beampreferably arrives at the area of the tissue at least substantiallysimultaneously. This provides for the most efficient ablation of thearea of the tissue.

The wavelength of the ultraviolet light may be in the range of between180 to 225 nm, and the wavelength of the infrared light may be in therange of between 1.4 to 11.0 μm and most preferably around 3.0 μm. Thisis the wavelength closest to the absorption peak of water and hencefacilitates the ablation of tissue.

A single first laser beam and/or a single second laser beam may beprovided. It is however envisaged that more than one first laser beamand/or second laser beam be provided.

The method may include shifting the frequency of a laser beam ofinfrared light into the ultraviolet range to thereby provide said firstlaser beam.

The method may further include producing said first and second laserbeams from a first and second laser means respectively, and preferablyrespectively smoothing, shaping and/or scanning said first and secondlaser beams.

The first and second laser beams may be mixed or combined to therebyprovide a combined laser beam for said ablation. The combined laser beammay also or alternatively be smoothed, shaped and/or scanned.

It is alternatively envisaged that the method may include directing thefirst and second laser beams from different directions to the ablationarea of the tissue.

It is also envisaged that the method may include producing a singlelaser beam from a laser means and splitting said single laser beam toprovide said first and second laser beams. The single laser beam may besmoothed, shaped and/or scanned.

According to another aspect of the present invention, there is providedan apparatus for laser ablation of a corneal tissue, including means forproviding at least one first pulsed unfocused laser beam of ultravioletlight, means for providing at least one second pulsed unfocused laserbeam of infrared light, and means for directing the first and secondlaser beams to the area of tissue to be ablated to thereby ablate thetissue. The ultraviolet light and the infrared light may havewavelengths in the range referred to above. Furthermore, a single firstlaser means and a single second laser means may be provided.

The means for providing the second laser beam may include a second lasermeans for emitting an infrared laser beam. The second laser means mayfor example be an Erbium:YAG, CO₂ or a Holmium:YAG laser.

In one preferred arrangement according to the present invention, themeans for providing the first laser beam may include a first laser meansfor emitting an ultraviolet laser beam. The first laser means may forexample be a mini excimer laser.

In an alternative preferred arrangement according to the presentinvention, the means for providing the first laser beams may include afirst laser means for emitting an infrared laser beam and frequencyshifting means for converting the infrared laser beam into theultraviolet means. The first laser means may for example be aNeodymium:YAG, Titanium:Sapphire, Alexandrite, Erbium:YLF, Erbium:YAG ora Holmium:YAG laser.

In a further preferred arrangement according to the present invention,the apparatus may include a single laser means for emitting an infraredlaser beam wherein the means for providing the first and second laserbeams includes a first beam splitter for splitting of the infrared laserbeam into two laser beam portions, one of the laser beam portionsproviding the second laser beam, and a frequency shifting means throughwhich the other laser beam portion can pass to thereby provide the firstlaser beam. The laser means may for example be an Erbium: YAG laser,Titanium:Sapphire, Alexandrite, Neodymium:YAG, Erbium:YLF or aHolmium:YAG laser.

In yet another preferred arrangement according to the present invention,the means for providing the first laser beam may include a first lasermeans for emitting an infrared laser beam, frequency shifting means forat least substantially converting the infrared laser beam into anultraviolet laser beam with a portion thereof remaining unshifted, beamsplitting means for splitting the unshifted portion of the laser beamfrom the frequency shifted portion thereof, the shifted portionproviding the first laser beam, the unshifted portion thereof pumping asecond laser means for providing the second laser beam. The first lasermeans may for example be a titanium sapphire laser. This laser may beflash lamped pumped. In addition, this laser may optionally be seeded bya short pulse laser. The second laser means may be an Erbium:YAG, CO₂ ora Holmium:YAG laser.

The means for directing the first and second laser beams may includebeam smoothing means for smoothing the first and second beamsrespectively, beam shaping means for shaping the first and second laserbeams respectively and/or beam scanning means for scanning the first andsecond laser beams respectively.

The above described apparatus may include means for combining the firstand second laser beams into a combined laser beam for directing to saidablation area. The combining means may include a dichroic mirror throughwhich one of said laser beams can pass therethrough and mirror means forreflecting the other of said laser beams to the dichroic mirror, wherebythe other said laser beam is reflected at least substantially in thesame direction as the said one laser beam to thereby provide saidcombined laser beams. Smoothing means may be provided for smoothing thecombined laser beam. Furthermore, beam shaping means may be provided forshaping the combined laser beam. In addition, beam scanning means may beprovided for scanning the combined laser beam.

The corneal tissue may be ablated for the purpose of producingrefractive corrections for an eye.

When the ultra-violet and infra-red laser beams are applied to thecorneal tissue, the photons of the laser beam of ultra-violet lightbreaks enough bonds of the corneal tissue to make the ablation processsufficiently efficient to limit damage to the surrounding tissue,whereas the photons of the laser beam of infra-red light provides enoughenergy to complete the ablation process.

The present invention will be more readily understood from the followingdescription of preferred practical arrangements of the apparatus of thepresent invention as illustrated in the accompanying drawings:

FIG. 1 is a flow diagram of a first arrangement of an apparatusaccording to the present invention;

FIG. 2 is a flow diagram of a second arrangement of an apparatusaccording to a present invention;

FIG. 3 is a flow diagram of a third arrangement of an apparatusaccording to the present invention; and

FIG. 4 is a flow diagram of a fourth arrangement of an apparatusaccording to the present invention.

Referring initially to FIG. 1, the first arrangement of the apparatusincludes an Erbium:YAG laser 1 and a Neodymium:YAG laser 2. TheErbium:YAG laser 1 is a low gain laser providing an infrared (IR) laserbeam 11 having a wavelength of 2.94 μm. This wavelength is closest tothe absorption peak of water at 3.0 μm and therefore able to ablatetissue very well. This IR laser beam 11 initially passes through beamsmoothing components 3 of the apparatus before passing through adichroic mirror 4 which allows transmission of IR light but reflectsultraviolet (UV) light.

The Neodymium:YAG laser 2 also produces an IR laser beam 12. This laser2 has high power lasing at a wavelength of 1.06 μm. The fifth harmonicof this laser 2 is 213 nm. It is therefore advantageous and relativelyconvenient to frequency shift the IR laser beam 12 produced by theNeodymium:YAG laser 2 closer to the desired UV range. This laser beam 12is therefore initially passed through frequency shifting components 6which increases the light frequency of the laser beam 12 by a multipleof 5 to thereby bring the laser beam 12 into the UV range. The frequencyshifting components 6 may comprise frequency conversion crystals thatcan frequency double the frequency of the laser beam 12 twice and thenmix with the original wavelength to form the fifth harmonic. Otherarrangements are also envisaged. The laser beam 12 then passes throughbeam smoothing components 7 which may for example comprise anarrangement having a spatial filter, optical integrator and/or an imagerotator. Such arrangements are used in excimer laser systems and willnot be described in detail herein. After passing through the beamsmoothing components 7, the laser beam 12 is then reflected off a mirror8 to a dichroic mirror 4. Because this mirror 4 reflects UV light, thislaser beam 12 is reflected in a direction at least substantiallyparallel to and along the same path as the laser beam 11 from theErbium:YAG laser 1. This results in at least a degree of mixing of thetwo laser beams to provide a combined laser beam 13. This combined beam13 passes through beam shaping components 5 which directs the combinedbeam 13 the corneal tissue of the eye 10. A computer 9 controls thevarious components of the apparatus, in particular the two lasers 1, 2and the beam shaping components 5. These components 5 may comprise amotorised iris diaphragm or masks and other arrangements are alsoenvisaged.

In the arrangement shown in FIG. 2, only one Erbium:YAG laser 20 isrequired in the apparatus. The IR laser beam 28 from the laser 20 passesthrough beam smoothing components 21 to a beam splitter 22. This beamsplitter 22 splits the IR laser beam 28 into two separate laser beams30, 29, one light beam 30 passing through frequency shifting components23 to convert the beam 30 into UV light. This UV laser beam 30 thenpasses through a dichroic mirror 24 which, unlike the dichroic mirror ofthe first arrangement, reflects IR light and transmits UV light.

The second IR laser beam 29 is reflected via mirrors 27 to the dichroicmirror 24 which reflects this laser beam 29 in parallel with and alongthe same path as the UV laser beam 30. As in the first arrangement, thetwo laser beams are least substantially mixed to provide a combined alaser beam 30a which passes through beam shaping components 25 to theeye 10. A computer 26 controls the operation of the laser 20 and thebeam shaping components 25.

In the arrangement shown in FIG. 3, there is provided a mini-excimerlaser 32 and a Erbium:YAG laser 31. Mini-excimer lasers use differenttechnology and are less powerful than the excimer lasers normally used.These lasers therefore do not provide enough energy to be used for laserablation. They are however significantly less expensive and only usesmall quantities of gas compared with the more powerful excimer lasers.As the gas used is very toxic and corrosive, mini-excimers are thereforesafer to use. The mini-excimer laser 32 provides a UV laser beam 36which passes through beam shaping components 33 directly to the eye 10.The IR laser beam 37 of the Erbium:YAG laser 31 also passes throughseparate beam shaping components 34 and directly to the eye 10. As thelaser beams come from different directions, there is no prior mixing ofthe laser beams prior to being applied to the eye in this arrangement.Both beam shaping components 34 are controlled by a computer 35.

In the arrangement shown in FIG. 4, there is provided a flash-lamppumped titanium sapphire laser 40. This laser 40 may optionally also beseeded by a short pulse diode laser (not shown). This provides a shorterpulse width for the emitted laser beam 44 resulting in higher peakpowers during the pulse, greater efficiency for the frequency conversionprocess into UV range and less damage to tissue adjacent to the ablationsite.

The laser beam 44 emitted by the titanium sapphire laser is typicallywithin the near IR range typically having a wavelength of between 700and 800 nm. This laser beam 44 passes through frequency shiftingcomponents 41, typically frequency conversion crystals, which twicedoubles the frequency of the laser beam 44 such that the laser beam 44aemitted from the frequency shifting components 41 is primarily in the UVrange having a wavelength of around 200 nm. A residual portion of thelaser beam 44a however remains unshifted in the near IR range.

A beam splitting arrangement 45, typically a dichroic mirror 45, splitsthe laser beam 44a into a UV laser portion 46 and the frequencyunshifted IR portion 47. The latter IR portion 47 pumps an Erbium:YAGlaser 42 which, as noted previously, emits a laser beam 50 in the IRrange having a wavelength of 2.94 μm.

The UV laser portion 46 is reflected via mirrors 48 to a second dichroicmirror 49. The laser beam 50 from the Erbium:YAG laser 42 is alsodirected to and passes through the second dichroic mirror 49.

The UV laser portion 46 is reflected by the second dichroic mirror 49along a path parallel to the Erbium:YAG laser beam 50 to provide acombined laser beam 51. This combined beam 51 passes through a beamsmoothing, shaping and scanning arrangement 43 before being directed tothe eye 10.

In all of the above arrangements, it is also envisaged that beamscanning means such as a mirror arrangement for scanning the IR and/orUV laser beam or the combined beam also be provided to operate inconjunction with the beam shaping components to scan the beam(s) to thecorrect position on the eye.

It is to be appreciated that alternative types of lasers are alsoapplicable for use in an apparatus according to the present inventionand that the present invention is not restricted to the use of the lasertypes hereinbefore referred to.

The apparatus of the present invention therefore provides for accurateablation with tissue damage about the same as a 193 nm excimer laserablation over a large area of the cornea (more than 1 mm² per pulse)without the need for a normal size excimer laser. This leads to areduction in overall size of the laser ablation apparatus as well as asignificant reduction in the maintenance and running costs for thisapparatus without compromising the clinical result.

I claim:
 1. A method for laser ablation of corneal tissue comprisingdirecting at least one first pulsed unfocused laser beam of ultravioletlight and at least one second pulsed unfocused laser beam of infraredlight to an area on the tissue to thereby ablate said tissue.
 2. Amethod according to claim 1 further comprising timing the pulses of eachsaid at least one first and second laser beams to arrive at the area ofthe tissue at least substantially simultaneously.
 3. A method accordingto claim 1 wherein the ultraviolet light has a wavelength of between 180to 225 nm.
 4. A method according to claim 1 wherein the infrared lighthas a wavelength of between 1.4 to 11.0 μm.
 5. A method according toclaim 4 wherein the wavelength of the infrared light is around 3.0 μm.6. A method according to claim 1 comprising providing a single saidfirst laser beam or a single said second laser beam.
 7. A methodaccording to claim 1 comprising shifting the frequency of a laser beamof infrared light into the ultraviolet range to thereby provide saidfirst laser beam.
 8. A method according to claim 1 comprising producingsaid first and second laser beams from a first and second laser meansrespectively.
 9. A method according to claim 8 further comprisingrespectively smoothing, shaping and/or scanning said first and secondlaser beams.
 10. A method according to claim 1 comprising combining saidfirst and second laser beams to thereby provide a combined laser beamfor said ablation.
 11. A method according to claim 10 further comprisingsmoothing, shaping and/or scanning said combined laser beam.
 12. Amethod according to claim 1 comprising directing the first and secondlaser beams from different directions to the ablation area of thetissue.
 13. A method according to claim 1 further comprising producing asingle laser beam from a laser means and splitting said single laserbeam to provide said first and second laser beams.
 14. A methodaccording to claim 13 comprising smoothing, shaping and/or scanning saidsingle laser beam.
 15. A method according to claim 14 comprising aablating the corneal tissue to thereby produce refractive correctionsfor an eye.
 16. A method according to claim 1 comprising providing asingle said first laser beam and a single said second laser beam.
 17. Anapparatus for laser ablation of a corneal tissue, comprising a means forproviding at least one first pulsed unfocused laser beam of ultravioletlight, a means for providing at least one second pulsed unfocused laserbeam of infrared light, and a means for directing the first and secondpulsed laser beams to the area of tissue to be ablated to thereby ablatesaid tissue.
 18. An apparatus according to claim 17 wherein the pulsesof each said at least one first and second laser beams arrives at thearea of the tissue at least substantially simultaneously.
 19. Anapparatus according to claim 17 wherein the ultraviolet light has awavelength of between 180 to 225 nm.
 20. An apparatus according to claim17 wherein the infrared light has a wavelength of between 1.4 to 11.0μm.
 21. An apparatus according to claim 19 wherein the wavelength of theinfrared light is around 3.0 μm.
 22. An apparatus according to claim 17wherein a single said first laser beam and/or a single said second laserbeam is provided.
 23. An apparatus according to claim 17 wherein saidmeans for providing the second laser beam includes a second laser meansfor emitting an infrared laser beam.
 24. An apparatus according to claim20 wherein the second laser means is an Erbium: YAG, CO₂ or aHolmium:YAG laser.
 25. An apparatus according to claim 17 wherein saidmeans for providing the first laser beam includes a first laser meansfor emitting an ultraviolet laser beam.
 26. An apparatus according toclaim 23 wherein the first laser means is a mini excimer laser.
 27. Anapparatus according to claim 17 wherein said means for providing thefirst laser beam includes a first laser means for emitting an infraredlaser beam and frequency shifting means for converting the infraredlaser beam into the ultraviolet means.
 28. An apparatus according toclaim 27, wherein the first laser means is a Neodymium:YAG,Titanium:Sapphire, Alexandrite, Erbium:YLF, Erbium:YAG or a Holmium:YAGlaser.
 29. An apparatus according to claim 17 further comprisingincluding a single laser means for emitting an infrared laser beamwherein the means for providing the first and second laser beamsincludes a first beam splitter for splitting of the infrared laser beaminto two laser beam portions, one of the laser beam portions providingthe second laser beam, and a frequency shifting means through which theother laser beam portion can pass to thereby provide the first laserbeam.
 30. An apparatus according to claim 28 wherein the laser means isan Erbium:YAG, Titanium:Sapphire, Alexandrite, Neodymium:YAG, Erbium:YLFor a Holmium:YAG laser.
 31. An apparatus according to claim 17 whereinthe means for providing the first laser beam comprises a first lasermeans for emitting an infrared laser beam, frequency shifting means forat least substantially converting the infrared laser beam into anultraviolet laser beam with a portion thereof remaining unshifted, beamsplitting means for splitting the unshifted portion of the laser beamfrom the frequency shifted portion thereof, the shifted portionproviding the first laser beam, the unshifted portion thereof pumping asecond laser means for providing the second laser beam.
 32. An apparatusaccording to claim 31 wherein the first laser means is a titaniumsapphire laser.
 33. An apparatus according to claim 32 wherein thetitanium sapphire laser is flashed pumped.
 34. An apparatus according toclaim 32 wherein the titanium sapphire laser is seeded by a short pulsediode laser.
 35. An apparatus according to claim 31 wherein the secondlaser means is an Erbium:YAG, CO₂ or a Holmium:YAG laser.
 36. Anapparatus according to claim 17 wherein the means for directing saidfirst and second laser beams includes beam smoothing means for smoothingthe first and second beams respectively.
 37. An apparatus according toclaim 17 wherein the means for directing said first and second laserbeams includes beam shaping means for shaping the first and second laserbeams respectively.
 38. An apparatus according to claim 17 wherein themeans for directing said first and second laser beams includes beamscanning means for scanning the first and second laser means.
 39. Anapparatus according to claim 17 means for combining the first and secondlaser beams into a combined laser beam for directing to said ablationarea.
 40. An apparatus according to claim 39 wherein the combining meanscomprises a dichroic mirror through which one of said laser beams canpass therethrough and mirror means for reflecting the other of saidlaser beams to the dichroic mirror, whereby the other said laser beam isreflected at least substantially in the same direction as the said onelaser beam to thereby provide said combined laser beams.
 41. Anapparatus according to claim 39 further comprising a including beamsmoothing means for smoothing the combined laser beam.
 42. An apparatusaccording to claim 38 further comprising a beam shaping means forshaping the combined laser beam.
 43. An apparatus according to claim 39further comprising a beam scanning means for scanning the combined laserbeam.
 44. An apparatus according to claim 17 wherein the apparatusablates the corneal tissue to thereby produce refractive corrections foran eye.